System.NotSupportedException : Values of type 'String' cannot be deserialized using a serializer of type 'CastingSerializer<String, IEnumerable<String>>'.
   в MongoDB.Bson.Serialization.Serializers.SerializerBase`1.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   в MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize[TValue](IBsonSerializer`1 serializer, BsonDeserializationContext context)
   в MongoDB.Bson.Serialization.Serializers.EnumerableSerializerBase`2.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   в MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize[TValue](IBsonSerializer`1 serializer, BsonDeserializationContext context)
   в MongoDB.Driver.Core.Operations.ElementDeserializer`1.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   в MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize[TValue](IBsonSerializer`1 serializer, BsonDeserializationContext context)
   в MongoDB.Driver.Core.WireProtocol.CommandWireProtocol`1.ProcessReply(ConnectionId connectionId, ReplyMessage`1 reply)
   в MongoDB.Driver.Core.WireProtocol.CommandWireProtocol`1.Execute(IConnection connection, CancellationToken cancellationToken)
   в MongoDB.Driver.Core.Servers.Server.ServerChannel.ExecuteProtocol[TResult](IWireProtocol`1 protocol, CancellationToken cancellationToken)
   в MongoDB.Driver.Core.Servers.Server.ServerChannel.Command[TResult](DatabaseNamespace databaseNamespace, BsonDocument command, IElementNameValidator commandValidator, Func`1 responseHandling, Boolean slaveOk, IBsonSerializer`1 resultSerializer, MessageEncoderSettings messageEncoderSettings, CancellationToken cancellationToken)
   в MongoDB.Driver.Core.Operations.CommandOperationBase`1.ExecuteProtocol(IChannelHandle channel, ServerDescription serverDescription, ReadPreference readPreference, CancellationToken cancellationToken)
   в MongoDB.Driver.Core.Operations.CommandOperationBase`1.ExecuteProtocol(IChannelSource channelSource, ReadPreference readPreference, CancellationToken cancellationToken)
   в MongoDB.Driver.Core.Operations.ReadCommandOperation`1.Execute(IReadBinding binding, CancellationToken cancellationToken)
   в MongoDB.Driver.Core.Operations.DistinctOperation`1.Execute(IReadBinding binding, CancellationToken cancellationToken)
   в MongoDB.Driver.OperationExecutor.ExecuteReadOperation[TResult](IReadBinding binding, IReadOperation`1 operation, CancellationToken cancellationToken)
   в MongoDB.Driver.MongoCollectionImpl`1.ExecuteReadOperation[TResult](IReadOperation`1 operation, ReadPreference readPreference, CancellationToken cancellationToken)
   в MongoDB.Driver.MongoCollectionImpl`1.ExecuteReadOperation[TResult](IReadOperation`1 operation, CancellationToken cancellationToken)
   в MongoDB.Driver.MongoCollectionImpl`1.Distinct[TField](FieldDefinition`2 field, FilterDefinition`1 filter, DistinctOptions options, CancellationToken cancellationToken)

[TestFixture]
public class DistinctValuesForAnArrayFieldTest
{
    [Test]
    public void ShouldDistinctValuesForAnArrayField()
    {
        // Given

        var mongoClientSettings = new MongoClientSettings { Server = new MongoServerAddress("localhost") };
        var mongoClient = new MongoClient(mongoClientSettings);
        var mongoDatabase = mongoClient.GetDatabase("test");
        var mongoCollection = mongoDatabase.GetCollection<News>("news");

        mongoCollection.InsertMany(new[]
                                   {
                                       new News { Title = "News 1", Tags = new [] { "A", "B" } },
                                       new News { Title = "News 2", Tags = new [] { "B", "C" } },
                                       new News { Title = "News 3", Tags = new [] { "C", "D" } }
                                   });

        // When

        // It works fine only for v2.4.0 and throws an exception for v2.4.1
        // System.NotSupportedException : Values of type 'String' cannot be deserialized using a serializer of type 'CastingSerializer<String, IEnumerable<String>>'.
        var uniqueTags = mongoCollection.Distinct<string>("Tags", Builders<News>.Filter.Empty).ToList();


        // Then

        CollectionAssert.AreEquivalent(new[] { "A", "B", "C", "D" }, uniqueTags);
    }
}


public class News
{
    public string Title { get; set; }

    public IEnumerable<string> Tags { get; set; }
}